This invention relates generally to air intakes for aircraft engines, and in particular to a barrier filter for a helicopter inlet. An air-breathing turbine or piston engine for aircraft propulsion requires intake air that is generally clean to provide for efficient combustion and to avoid internal damage. The engine is designed with small tolerances between moving parts to maximize efficiency, but which also increase vulnerability to damage from small contaminants. Unfortunately, helicopters operate at low altitudes where intake air can be contaminated with material from the ground, such as sand and dust. Rotor downwash aggravates that problem and moreover causes articles such as leaves, dry grass, and debris on the ground to become airborne where they can be ingested by the engine. Contamination of intake air, even in a small amount, causes premature wear on engine components, increases maintenance costs, and degrades operational reliability.
Systems which remove contaminants from intake air flow have been developed to protect the engine from damage. A first type of system is an inertial particle separator, which employs the momentum of each moving contaminant particle to separate it from intake air. Axial flow inertial particle separators (see, e.g., U.S. Pat. No. 5,139,545) have curved intake ducts which direct particles, due to their momentum, into a scavenge area on an outward part of a curved turn, leaving clean air on the inward part of the turn to enter the engine. Vortex flow inertial particle separators (see, e.g., U.S. Pat. No. 3,449,891) have a plurality of tubes with helical vanes which swirl the flow to create centrifugal forces, deflecting particles outwardly for disposal.
Ideally, these systems should remove all contaminants from the intake air flow. One drawback to inertial particle separators is limited effectiveness in removing particles, particularly those of the smallest size. Typical separators remove from 65% to 85% of the contaminants, which can be unacceptably low. Further, these systems should introduce minimal loss of pressure to the intake air as it flows through, and minimal non-uniformity to pressure with aircraft flight speed or direction. High inlet pressure is preferred for good engine performance and uniform for good operational stability of the engine. Unfortunately, some separators become plugged by larger contaminants, which degrades effectiveness and causes a large decrease in inlet pressure. These separators are also difficult to clean when they are plugged.
A second type of system for removing contaminants is a porous barrier filter. The filter is positioned such that in normal operation, all intake air must flow through the filter prior to reaching the engine. The filter is highly effective in removing particles of all sizes and offers cost and performance benefits over inertial type separation.
Unfortunately, integration of barrier filters into aircraft poses a number of difficulties, especially into aircraft that are not originally designed for these systems. Barrier filters must be properly sized to permit adequate quantity of air to flow through without a large pressure drop across the filter. Barrier filters should also be located where re-ingestion of exhaust gas into intake air is avoided, and for military aircraft, avoiding ingestion of exhaust gas from weapons which are fired. The installation of barrier filters should cause minimal change to aircraft external surface contours. Changes to external contours of existing, previously flight certified aircraft could require requalification and recertification testing of the aircraft for operability, performance and handling characteristics. That testing can be expensive and time consuming.
Barrier filters and inlets in which they operate should be located on the aircraft where pressure of the intake air flowing into the engine is relatively independent of directional motion of the aircraft. Otherwise, a change in direction of flight can produce engine instability and performance degradation. For example, a barrier filter positioned in an inlet facing forwardly receives air at an elevated pressure when the helicopter is moving in a forward direction. The increase, or xe2x80x9cramxe2x80x9d pressure, is favorable and is the result of converting the momentum of the higher velocity air to a higher pressure and lower velocity, as it flows into the inlet plenum. However, when the helicopter stops moving in a forward direction, the ram pressure is lost. This lower pressure results in a decrease in engine efficiency which is detrimental to stable operation of the engine.
Among the several objects and features of the present invention that may be noted and the provision of an aircraft engine intake air filtration system which effectively removes contaminants; the provision of such a system which is usable in existing aircraft without modifications to external surface contours; the provision of such a system which can replace previously installed particle separators; the provision of such a system which is conformal to the aircraft external surface contours; the provision of such a system which minimizes loss and non-uniformity of pressure to the intake flow; and the provision of such a system which is positioned to provide intake air having a pressure independent of directional motion of the aircraft.
Generally, an air induction system for a helicopter according to the present invention receives intake air, removes contaminants from the intake air, and provides the intake air for delivery to an engine at a pressure which is substantially independent of whether the helicopter is moving in a forward direction or hovering with no forward motion. The helicopter has a longitudinal axis and a generally horizontal rotor. The system comprises a first entryway for receiving intake air, the first entryway positioned on the helicopter generally facing the forward direction and perpendicular to the longitudinal axis such that forward motion of the helicopter directs intake air to flow directly into the first entryway with an elevated pressure due to the forward motion. A first barrier filter is mounted across the first entryway, the first filter having a porous media and positioned such that all air received in the first entryway flows to the first filter. A second entryway receives intake air, the second entryway being positioned below the rotor and facing generally upwardly such that during hovering flight, the rotor directs intake air downwardly to flow directly into the second entryway with an elevated pressure due to downward motion. A second barrier filter is mounted across the second entryway, the second filter having a porous media and positioned such that all air received in the second entryway flows to the second filter.
In another aspect, a helicopter according to the present invention has an air filtration system to remove contaminants from intake air prior to delivery to an engine. The helicopter has a longitudinal axis. The helicopter comprises an intake for receiving the intake air and a peripheral external surface adjacent to the intake. The peripheral external surface has a smooth, continuous contour in the longitudinal direction which facilitates, during forward flight of the helicopter, generally smooth, streamlined flow of external air adjacent the intake to minimize aerodynamic drag. A barrier filter is mounted in the intake, the filter having a porous filter element. The barrier filter has an upstream side and a downstream side, the upstream side being mounted flush across the intake such as to meet the contour of the peripheral external surface.
In yet another aspect, a method according to the present invention retrofits a helicopter with an improved system for removing contaminants from intake air prior to delivery to an engine. The helicopter has a fuselage with at least two openings for admitting intake air into the fuselage, a surface surrounding each opening with a smooth, continuous contour adjacent to each opening, and an inertial particle separator mounted in each of the openings to remove contaminants from the intake air. The helicopter has an initial external moldline. The method comprises removing the inertial particle separators from each of the openings in the fuselage, and sizing at least two pleated barrier filters for the openings such that a calculated speed of intake air flowing therethrough, when the engine is operating at take off power (TOP) or military intermediate rated power (IRP), is less than about 30 feet/second. The pleated barrier filters are mounted in corresponding openings in the fuselage, each filter having an upstream surface and a downstream surface. The filter is mounted such that the upstream side meets and substantially conforms to the contour of the surface such that the mounting does not alter the initial external moldline of the helicopter.
Other objects and features will be in part apparent and in part pointed out hereinafter.